1. Field of the Invention
The present invention relates to a temperature detector using crystal resonators and a temperature compensated oscillator having the temperature detector.
2. Description of the Prior Art
A temperature compensated oscillator has a voltage controlled oscillator (VCO) which is a main oscillator, a temperature detector for detecting the temperature around the main oscillator, and a control voltage generating circuit for generating a control voltage from the detected temperature, which maintains the oscillation frequency of the VCO constant by compensating for the changes in ambient temperature and applying such a control voltage to the VCO. The temperature detector has an oscillator with a piezoelectric resonator whose oscillation frequency varies with its temperature. Temperature is detected in terms of the oscillation frequency of the oscillator.
One example of such a temperature detector using the combination of an AT cut and a Y cut crystal resonators is described in a paper entitled "LOW PROFILE HIGH STABILITY DIGITAL TCXO; ULTRA LOW POWER CONSUMPTION TCXO" by V. Candelier et al., Proceedings of the 43rd Annual Symposium on Frequency Control, 1989, pp. 51-54. An AT cut crystal resonator has a small temperature coefficient of natural oscillation frequency while a Y cut crystal resonator has a large temperature coefficient of natural oscillation frequency. In such a case, the temperature detector counts the output pulses of the oscillator implemented by the AT cut crystal resonator while the output of the oscillator implemented by the Y cut crystal resonator is in a high level. The temperature detector resets the count to zero every predetermined period while delivering the count at the time just before resetting as a detected temperature signal.
Another example of the temperature detector using a single crystal resonator is disclosed in a paper entitled "FACTORS INFLUENCING STABILITY IN THE MICROCOMPUTER-COMPENSATED CRYSTAL OSCILLATOR" by A. Benjaminson, Proceedings of the 44th Annual Symposium on Frequency Control, 1990, pp. 597-614. This detector has a single SC cut crystal resonator for temperature detection and excites the resonator by two oscillation circuits assigned to, respectively, a fundamental harmonic and a higher harmonic. As a result, the detector generates oscillator outputs, one is a fundamental harmonic and the other is a tertiary higher harmonic. A signal representative of a frequency difference between the two oscillation outputs is used as a detected temperature signal.
The problem with the above-described dual resonator scheme is that the temperature detection error increases transiently when the temperature is sharply changed due to the turn-on of a power source or a sharp change in ambient temperature. Specifically, to enhance accurate temperature detection, a large ratio is usually selected between the natural frequency (fA) of the AT cut resonator and the natural frequency (fY) of the Y cut resonator (e.g. fA=1 MHz and fY=10 kHz). As a result, the Y cut resonator has a far larger volume and a far larger heat capacity than those of the AT cut resonator. Therefore, the two resonators are noticeably different in thermal time constant. When the temperature is sharply changed, a temperature detection error ascribable to the temperature difference between the two resonators is not avoidable during the transient period to a steady temperature state.
On the other hand, the single SC cut crystal resonator scheme is free from the temperature detection error ascribable to the difference in thermal time constant. However, this conventional scheme has a drawback that the oscillation circuit for generating the tertiary higher harmonic increases power consumption. Specifically, the power consumption by the semiconductor devices constituting the oscillation circuits and the entire circuitry including them increases substantially in proportion to oscillation frequency. Hence, the oscillation circuit assigned to the tertiary higher harmonic consumes almost three times greater power than the oscillation circuit assigned to the fundamental harmonic. It follows that the total power consumption is almost four times greater than that of the oscillation circuit assigned to the fundamental harmonic.